Matrix is the protocol. Element is one of the (many) clients. Setting up an account on a server is as easy or easier than discord. Try it app.element.io
Matrix has video and voice rooms, screen-sharing, direct calls, threads, and very little fluff. An entire conference (FOSDEM) was hosted on a matrix server and people from any homeserver could connect. Admittedly, I don’t use other features, but those are all that I need. What other features are essential for an opensource community that only discord provides?
As for forums, they are for async. Are you going to seriously tell me discord is a good forum replacement?